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Nervous System
Nervous System Overview
Sensory input
Sensory
receptors
Collect info
Integration
Interpretation
of stimuli
Occurs in CNS (brain & spinal
cord)

Nervous System Overview


Motor Output
 Conduction of signal from CNS to
effector cells
 Carry out response to stimuli
 Muscles & glands
Signal conducted by
nerves
 CNS
 PNS (outside of CNS)
Nervous System Overview
Nervous System Overview

Nerve Circuit
Simplest: automatic (reflex arc)
Sensory neuron interneuron of
spinal cord  motor neuron 
effector cell
Nervous System Overview
Ganglion: cluster of nerve cell
bodies with similar functions in
PNS
Cluster within brain: nuclei
Supporting cells :glia
“Glue” neurons
together
Helps embryonic
development of CNS

Nervous System Overview
Astrocytes: provide support for
neurons, induce tight junction
formation for cells lining capillaries of
brain
Blood-brain barrier
Restricts passage
of substances into
brain

Nervous System Overview
Oligodendrocytes (in CNS)
[Schwann cells in PNS]
Form insulating lipid sheath
around neuron axon
Increase
speed of
nerve
impulse
Deterioration
of: MS

Membrane Potential
Nerve signals: changes in voltage
across plasma membrane of nerve cells
Caused by ion movement
More anions inside; more cations
outside
Membrane now electrically
polarized
-50 to –100 mV in resting state
(unstimulated)

Membrane Potential
Inside: cations are mostly K+, some
Na+ (anions are proteins, a.a.,
sulfate, P, Cl-)
Outside: cations are mostly Na+,
some K+ (anions are mostly Cl-)
Ions only pass through selective
protein channels
Most cells more permeable to K+
than Na+
Internal anions too large to cross

Membrane Potential
Tendency for K+ to diffuse out, Na+
in
Through channels that are always
open (ungated)
Sodium-potassium pump uses ATP
to bring K+ in & Na+ out
Maintains ionic gradient – keep
potential

Membrane Potential
Cells that generate changes in
potential – excitable cells
When unexcited: resting potential
When stimulated, gated ion
channels open - allow specific ion
through

Membrane Potential
Some stimuli trigger
hyperpolarization – inc in voltage
across membrane (K+ out  cell
more negative)
Some trigger depolarization (Na+ in
 cell less negative)
Strong enough stimuli trigger action
potential (nerve impulse)

Action Potential

All or nothing

Few milliseconds

In between stimuli: refractory period

More intense stimuli – repeated action potentials
with refractory period in between

Action potential at one point of axon triggers next
action potential
Action Potential

Speed of action potential influenced by:

Diameter of axon (larger = faster)

Myelination of axon

Action potential only occurs at nodes
(between Schwann cells)

Appears to “jump” – saltatory
conduction
Synapses

Cell junctions between neurons, receptors,
effectors

Two types

Electrical synapses

Action potential spreads directly from
cell to cell

Cells connected by gap junctions
Synapses

Chemical synapses

Gap (synaptic cleft) separates cells

Action potential cannot be directly
transmitted

Conversion of electrical signal to chemical
signal that bridges gap

Action potential at end of neuron releases
Ca2+ into neuron
Synapses
This causes synaptic vesicles to release
chemicals (neurotransmitters) into
synaptic cleft
 Receptors on next neuron receive
chemicals – opens ion channels –
allows K+, Na+, Cl- across – impulse
continues

Neurotransmitters

Figure 48.1
Nervous System Diversity
Vertebrate Nervous System